The present invention relates to a correction method and printing apparatus and, more particularly, to a correction method and printing apparatus for performing printing using a printhead based on an inkjet printing method.
In a printing apparatus for performing printing by discharging an ink to printing paper while serially scanning a printhead according to an inkjet printing method, the position of the printhead at the start of discharge and the position at which the ink reaches the printing paper do not match generally because the ink is discharged while moving the printhead. This will be called a printing position shift herein.
If printing is performed in only one direction, the printing position shift basically poses no problems. However, in reciprocal scanning of the printhead, i.e., when printing is performed in two directions for forward scanning and backward scanning, certain control is necessary. Otherwise, the printing positions in both directions never match, resulting in poor print image quality.
For an inkjet printer, not only an increase in printing speed but also an improvement in print quality is required. It is therefore important to print a high-quality image even in printing by moving the printhead in two direction.
In actual printing, to correct the printing position shift in the two directions, the position of the printhead at the start of discharge is shifted in advance with respect to the printhead moving direction, thereby reducing the printing position shift.
The one-direction (printhead scanning direction) component (xcex94X) of a printing position shift in bidirectional printing is determined by (1) ink discharge velocity (VDROP), (2) ink discharge angle (xcex8), (3) distance (L) between the printhead and the printing medium surface, and (4) printhead moving speed (VCR), as shown in FIG. 10, and calculated by equation (1):
xcex94X={L/(VDROPxc3x97sin xcex8)}xc3x97VCRxe2x80x83xe2x80x83(1)
Referring to FIG. 10, the forward head direction means a direction in which the printhead leaves its home position, and the backward head direction means a direction in which the printhead approaches its home position.
These factors generally have variations of individual devices and therefore cannot always have designed deviations. The deviation of the printing position shift in two directions due to the variations of the individual devices must be compensated for. An adjustment operation of compensating for the individual difference will be called bidirectional registration adjustment.
These variations occur due to mechanical factors, mechanical control factors, characteristic factors of printheads, and the like. For an actual device, (1) adjustment considering mechanical and control factors is performed in the manufacturing process at factories, and (2) adjustment considering the printhead characteristic factors is performed by end users.
Conventional bidirectional registration adjustment is realized by simply changing the printing position shift amount.
More specifically, adjustment is performed in accordance with the following procedures.
(1) For each variation factor, a printing position shift amount (default correction value) is calculated from the design value on the basis of equation (1) and stored in a ROM in advance.
(2) In the print head manufacturing process at a factory, a deviation from the default correction value is stored in a non-volatile memory (NVRAM) as a device variation and used as a factory correction value.
(3) Finally, an end user stores a deviation from the default correction value+factory correction value in an NVRAM as a printhead variation and used as a user correction value.
(4) After this, printing is performed by using the default correction value+factory correction value+user correction value as a printing position shift amount.
In (2) and (3), (a) first, adjustment patterns obtained by changing the shift amount at a predetermined interval are printed on a printing medium, (b) the printed adjustment patterns are visually compared and checked, and an adjustment value for obtaining a printing result which is supposed to be the best pattern is selected, and (c) finally, the selected adjustment value is stored in the NVRAM, thereby obtaining the correction value.
However, the conventional correction method assumes that the ink discharge velocity does not largely vary. That is, the variation in ink discharge velocity is not included in the factors taken into consideration for correction. For this reason, when the ink discharge velocity largely varies, the shift cannot be satisfactorily corrected.
Accordingly, it is an object of the present invention to provide a correction method and printing apparatus capable of performing satisfactory correction even though the ink discharge velocity varies due to various factors for changing the characteristics of a printhead, including a change in printhead over time and exchange of the printhead.
According to one aspect of the present invention, the foregoing object is attained by providing a correction method of correcting a printing position shift when printing on a print medium is performed by discharging ink from a printhead while reciprocally scanning the printhead, comprising: a test printing step of test-printing a plurality of patterns while changing printhead scanning speed; a selection step of selecting one of the plurality of printed patterns; and a calculation step of calculating a correction amount for correcting the printing position shift, based on the selected pattern.
Note that an operator makes a selection in the selection step.
It is preferable that the calculation step calculates the correction amount, based on information on an ink discharge velocity from the printhead used for test-printing the selected pattern. Furthermore, it is preferable that the above-described method further comprises a storage step of storing the information into a memory such as a non-volatile memory after performing the selection step, and wherein the calculation step calculates the correction amount, based on the information stored in the memory.
The printing position shift is a shift (xcex94X) with respect to a scanning direction of the printhead. The calculation step calculates the correction amount using an equation xcex94X={L/(VDROPxc3x97sin xcex8)}xc3x97VCR, where VDROP is the ink discharge velocity, VCR is the printhead scanning speed, L is a distance between the printhead and the printing medium, and xcex8 is an ink discharge angle with respect to the printing medium.
The test printing step preferably comprises tentatively performing printing while changing the printhead scanning speed by a predetermined amount.
According to another aspect of the present invention, the foregoing object is attained by providing a printing apparatus, having scanning means for reciprocally scanning a printhead, for performing printing on a print medium by discharging ink from the printhead while scanning the printhead by the scanning means, comprising: test pattern printing means for test-printing a plurality of patterns while changing a printhead scanning speed; selection means for selecting one of the plurality of patterns; calculation means for calculating a correction amount for correcting a printing position shift when printing on a print medium is performed by discharging ink from the printhead, based on a pattern selected from the plurality of patterns by the selection means; and printing means for performing actual printing while performing correction based on the correction amount calculated by the calculation means.
Note that the calculation means may calculate the correction amount, based on an ink discharge velocity from the printhead corresponding to a printhead scanning speed used for test-printing the pattern selected by the selection means.
It is preferable that the apparatus further comprises memory means for storing information on the ink discharge velocity, wherein the calculation means calculates the correction amount, based on the information stored by the memory means.
It is also preferable that the apparatus further comprises a scanning speed detection means for detecting a scanning speed during reciprocal scanning of the printhead, wherein the calculation means calculates the correction amount, based on the scanning speed detected by the scanning speed detection means and an ink discharge velocity from the printhead corresponding to a printhead scanning speed used for test-printing the pattern selected by the selection means.
The apparatus preferably further comprises a position detection means for detecting a position of the printhead during the reciprocal scanning.
The scanning speed detection means and the position detection means respectively detect the scanning speed and position of the printhead, based on an encoder signal obtained from a linear encoder placed along a scanning direction of the scanning means. The linear encoder can be an optical encoder or a magnetic encoder.
The printhead preferably comprises an inkjet printhead for discharging the ink to perform printing. The printhead preferably comprises an electrothermal transducer for generating a thermal energy to be applied to the ink, thereby discharging the ink using the thermal energy.
The memory means comprises a non-volatile memory such as an EEPROM.
In accordance with the present invention as described above, a plurality of patterns are test-printed while changing the printhead scanning speed, one of the plurality of printed patterns is selected, the correction amount for correcting the printing position shift is calculated, based on the selected pattern.
The invention is particularly advantageous since high-quality printing can be performed while correcting the printing shift even though the ink discharge velocity from the printhead changes.
With this arrangement, even though the quality of a manufactured printhead varies during the printhead manufacturing process, satisfactory correction can be performed to maintain high image quality. This contributes to increasing the yield, so improvement of productivity or reduction of production cost can be achieved.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.